1. Field of the Invention
The present invention is generally directed to a wireless communication systems and, more specifically, to the transmission of acknowledgement information in an uplink of a communication system.
2. Description of the Art
A communication system includes a DownLink (DL) that conveys transmission signals from a Base Station (BS), or NodeB, to User Equipment (UE) and an UpLink (UL) that transmits signals from UEs to the NodeB. A UE, also commonly referred to as a terminal or a mobile station, may be fixed or mobile and may be a wireless device, a cellular phone, a personal computer device, a mobile electronic device, or any other similar fixed or mobile electronic device. A NodeB is generally a fixed station and may also be referred to as an access point or some other equivalent terminology.
More specifically, the UL transmits data signals carrying information content, of control signals providing control information associated with the transmission of data signals in the DL, and of Reference Signals (RSs), which are commonly referred to as pilot signals. The DL also conveys transmissions of data signals, control signals, and RSs.
UL data signals are transmitted through a Physical Uplink Shared CHannel (PUSCH) and DL data signals are conveyed through a Physical Downlink Shared CHannel (PDSCH). In a case where a PUSCH transmission does not occur, a UE conveys UL Control Information (UCI) through a Physical Uplink Control CHannel (PUCCH). However, when a PUSCH transmission occurs, a UE may convey UCI together with data through the PUSCH.
DL control signals may be broadcast or sent in a manner that is UE-specific. Accordingly, UE-specific control channels can be used, among other purposes, to provide UEs with Scheduling Assignments (SAs) for PDSCH reception, or in other words, a DL SA, or a PUSCH transmission, or in other words, a UL SA. The SAs are transmitted from the NodeB to respective UEs using DL Control Information (DCI) formats through respective Physical DL Control CHannels (PDCCHs).
The NodeB may configure a UE through higher layer signaling, such as Radio Resource Control (RRC) signaling, a PDSCH and a PUSCH Transmission Mode (TM). The PDSCH TM or PUSCH TM is respectively associated with a DL SA or a UL SA and defines whether the respective PDSCH or PUSCH conveys one data Transport Block (TB) or two data TBs.
PDSCH or PUSCH transmissions are either scheduled to be assigned to a UE by the NodeB through higher layer signaling or through physical layer signaling, such as PDCCH signaling, using a respective DL SA or UL SA, or correspond to non-adaptive retransmissions for a given Hybrid Automatic Repeat reQuest (HARQ) process. Scheduling by higher layer signaling is referred to as Semi-Persistent Scheduling (SPS), and scheduling by PDCCH is referred to as dynamic scheduling. A PDCCH may also be used to release a SPS PDSCH or a SPS PDSCH. If a UE misses a PDCCH, or in other words, fails to detect a PDCCH, it also misses the associated PDSCH or PUSCH. This event will be referred to as a Discontinuous Transmission (DTX).
The UCI includes ACKnowledgment (ACK) information associated with a HARQ process, i.e., a HARQ-ACK. The HARQ-ACK information may consist of multiple bits corresponding to positive ACKs for TBs the UE correctly received or negative acknowledgements (NACKs) for TBs the UE incorrectly received. In a case where a UE does not receive a TB, it may transmit a DTX, which includes tri-state HARQ-ACK information, or both the absence and the incorrect reception of a TB can be represented by a NACK (in a combined NACK/DTX state). One consequence of a UE not conveying a DTX to the NodeB is that Incremental Redundancy (IR) cannot be used for its HARQ process. This leads to throughput loss. Another consequence is that PDCCH power control, based on DTX feedback, is not possible.
In Time Division Duplex (TDD) systems, DL and UL transmissions occur in different Transmission Time Intervals (TTIs) which are referred to as subframes. For example, in a frame comprising of 10 subframes, some subframes may be used for DL transmissions and some may be used for UL transmissions.
FIG. 1 illustrates a frame structure for a TDD system according to the related art.
Referring to FIG. 1, a 10 millisecond (ms) frame consists of two identical 5 ms half-frames. Each 5 ms half-frame 110 is divided into 8 slots 120 and 3 special fields: a DL Pilot Time Slot (DwPTS) 130, a Guard Period (GP) 140, and an UL Pilot Time Slot (UpPTS) 150. The length of DwPTS+GP+UpPTS is one subframe 160 and is 1 ms long. The DwPTS may be used for the transmission of synchronization signals from the NodeB while the UpPTS may be used for the transmission of random access signals from UEs. The GP facilitates the transition between DL and UL transmissions by absorbing transient interference.
The number of DL subframes and the number of UL subframes per frame can be different from each other and multiple DL subframes may be associated with a single UL subframe. The association between the multiple DL subframes and the single UL subframe is in the sense that HARQ-ACK information of bits generated in response to PDSCH receptions (which are data TBs) in the multiple DL subframes needs to be transmitted in the single UL subframe. This number of DL subframes is referred to as the bundling window and, in the example of FIG. 1, it is usually smaller than or equal to 4 subframes and it is always smaller than or equal to 9 subframes.
One method for a UE to convey HARQ-ACK information in a single UL subframe, in response to receiving PDSCHs in multiple DL subframes, is HARQ-ACK bundling where the UE transmits an ACK only if it correctly receives all data TBs, otherwise, the UE transmits a NACK. Therefore, HARQ-ACK bundling results in unnecessary retransmissions and reduced DL throughput as a NACK is transmitted even when a UE incorrectly receives only one data TB and correctly receives all other data TBs.
Another method for a UE to convey HARQ-ACK information in a single UL subframe, in response to receiving data TBs in multiple DL subframes, is HARQ-ACK multiplexing, which is based on PUCCH resource selection.
Yet another method for a UE to convey HARQ-ACK information in a single UL subframe, in response receiving data TBs in multiple DL subframes, is joint coding of the HARQ-ACK bits using, for example, a block code such as the Reed-Mueller (RM) code, which will be described below. The primary focus of the descriptions herein is on joint coding of HARQ-ACK bits. Although the transmission of HARQ-ACK information was described for brevity only for a PUCCH, the coding method is fundamentally the same for transmission in a PUSCH.
If a PDSCH conveys one TB, the respective HARQ-ACK information consists of one bit which is encoded as a binary ‘1’ if the TB is correctly received, such that the binary ‘1’ indicates an ACK, and is encoded as a binary ‘0’ if the TB is incorrectly received, such that the binary ‘0’ indicates a NACK. If a PDSCH conveys two TBs, in accordance with the Single User-Multiple Input Multiple Output (SU-MIMO) transmission method with a rank higher than one, the HARQ-ACK information consists of two bits [o0ACK o1ACK] with o0ACK corresponding to the first TB and o1ACK corresponding to the second TBmax If a UE applies bundling in the spatial domain, it generates only one HARQ-ACK bit. The transmission of one HARQ-ACK bit may use repetition coding and the transmission of two HARQ-ACK bits may use a (3, 2) simplex code.
FIG. 2 illustrates a PUSCH transmission structure according to the related art.
Referring to FIG. 2, the subframe 210 includes two slots. Each slot 220 includes NsymbUL symbols used to transmit data, a HARQ-ACK, or a RS. Each symbol 230 includes a Cyclic Prefix (CP) to mitigate interference due to channel propagation effects. The PUSCH transmission in one slot may be either at a same BandWidth (BW) or at a different BW than in the other slot. Some symbols in each slot are used to transmit RS 240, which enables channel estimation and coherent demodulation of the received data and/or HARQ-ACK information. The transmission BW consists of frequency resource units which will be referred to as Physical Resource Blocks (PRBs). Each PRB includes NscRB sub-carriers, or Resource Elements (REs), and a UE is allocated MPUSCH PRBs 250 for a total of MscPUSCH=MPUSCH·NscRB REs for the PUSCH transmission BW.
The last subframe symbol may be used for transmitting a Sounding RS (SRS) 260 from one or more UEs. The SRS provides the NodeB an estimate of the channel medium the respective UE experiences over the SRS transmission BW. The NodeB configures to each UE the SRS transmission parameters through higher layer signaling such as RRC signaling. The number of subframe symbols available for data transmission is NsymbPUSCH=2·(NsymbUL−1)−NSRS, where NSRS=1 if the last subframe symbol is used for SRS transmission and NSRS=0 otherwise.
Each RS or SRS is assumed to be constructed using a Constant Amplitude Zero Auto-Correlation (CAZAC) sequence. Orthogonal multiplexing of CAZAC sequences can be achieved by applying different Cyclic Shifts (CSs) to the same CAZAC sequence.
FIG. 3 illustrates a transmitter for transmitting data and HARQ-ACK in a PUSCH according to the related art.
Referring to FIG. 3, encoded HARQ-ACK bits 320 are inserted by puncturing encoded data bits 310 by the data puncturing unit 330. A Discrete Fourier Transform (DFT) is then performed by the DFT unit 340. The REs for the PUSCH transmission BW are selected by the sub-carrier mapping unit 350 as instructed from a controller 355. An Inverse Fast Fourier Transform (IFFT) is performed by an IFFT unit 360, CP insertion is performed by a CP insertion unit 370, and time windowing is performed by a filter 380, thereby generating a transmitted signal 390. For brevity, the encoding and modulation processes and additional transmitter circuitry such as a digital-to-analog converter, analog filters, amplifiers, and transmitter antennas are not illustrated.
The PUSCH transmission is assumed to be over a single cluster 395A or over multiple clusters 395B of contiguous REs in accordance to the DFT Spread Orthogonal Frequency Division Multiple (DFT-S-OFDM) method for signal transmission.
FIG. 4 illustrates a receiver for receiving a transmission signal as illustrated in FIG. 3 according to the related art.
Referring to FIG. 4, an antenna receives a Radio-Frequency (RF) analog signal and after further processing by units such as filters, amplifiers, and analog-to-digital converters, which are not shown for the purpose of brevity, a received digital signal 410 is filtered by a filter 420 for time windowing and the CP is removed by CP removal unit 430. Subsequently, the receiver unit applies a FFT by an FFT unit 440, selects the REs used by the transmitter by sub-carrier de-mapping by a sub-carrier demapping unit 450 under a control of controller 455. Thereafter, an Inverse DFT (IDFT) unit 460 performs an IDFT, an extraction unit 470 extracts the HARQ-ACK bits and places erasures at the respective REs for the data bits, and finally generates the data bits 480.
Assuming for simplicity that the PUSCH conveys a single data TB, for HARQ-ACK transmission in a PUSCH a UE determines the respective number of encoded HARQ-ACK symbols as shown in Equation (1)
                              Q          ′                =                  min          (                                    ⌈                                                                    O                                          HARQ                      -                      ACK                                                        ·                                      β                    offset                                          HARQ                      -                      ACK                                                                                                            Q                    m                                    ·                  R                                            ⌉                        ,                          4              ·                              M                sc                PUSCH                                              )                                    Equation        ⁢                                  ⁢                  (          1          )                    
In Equation (1), OHARQ-ACK is a number of HARQ-ACK information bits, also referred to as a HARQ-ACK payload, βoffsetHARQ-ACK is a parameter that the NodeB conveys to the UE through higher layer signaling, Qm is a number of data information bits per modulation symbol (Qm=2, 4, 6 for Quadrature Phase Shift Keying (QPSK), Quadrature Amplitude Modulation (QAM) 16, QAM64, respectively), R is a data code rate of an initial PUSCH transmission for the same TB, MscPUSCH is a PUSCH transmission BW in a current subframe, and ┌ ┐ is the ceiling function that rounds a number to a next integer.
The data code rate is defined as shown in Equation (2)
                    R        =                              (                                          ∑                                  r                  =                  0                                                  CB                  -                  1                                            ⁢                              K                r                                      )                    /                      (                                          Q                m                            ·                              M                sc                                  PUSCH                  -                  initial                                            ·                              N                symb                                  PUSCH                  -                  initial                                                      )                                              Equation        ⁢                                  ⁢                  (          2          )                    
In Equation (2), CB is a total number of data code blocks, Kr is a number of bits for a data code block number r, NsymbPUSCH-initial is a number of subframe symbols for the initial PUSCH transmission of the same TB and MscPUSCH-initial is a number of respective REs for the PUSCH transmission BW. The maximum number of encoded HARQ-ACK symbols is limited to the number of REs in 4 DFT-S-OFDM symbols (4·MscPUSCH) which may be located in the two subframe symbols adjacent to the RS in each of the two subframe slots, as shown in FIG. 2. The determination of the number of encoded HARQ-ACK symbols in a case where a PUSCH conveys multiple TBs, using for example the SU-MIMO transmission method, is similar to the case where a PUSCH conveys one TB, and thus, a respective description is omitted for brevity.
FIG. 5 illustrates a PUCCH structure in one subframe slot for the transmission of multiple HARQ-ACK information bits using the DFT-S-OFDM transmission method according to the related art.
Referring to FIG. 5, after encoding and modulation, using respectively, for example, a RM block code and QPSK (not shown for brevity), a set of the same HARQ-ACK bits 510 is multiplied by mixer 520 with elements of an Orthogonal Covering Code (OCC) 530 and is subsequently DFT precoded by the precoder unit 540. For example, for 5 symbols per slot carrying HARQ-ACK bits, the OCC has a length of 5 {OCC(0), OCC(1), OCC(2), OCC(3), OCC(4)} and can be either of {1, 1, 1, 1, 1}, or {1, exp(j2π/5), exp(j4π/5), exp(j6π/5), exp(j8π/5)}, or {1, exp(j4π/5), exp(j8π/5), exp(j2π/5), exp(j6π/5)}, or {1, exp(j6π/5), exp(j2π/5), exp(j8π/5), exp(j4π/5)}, or {1, exp(j8π/5), exp(j6π/5), exp(j4π/5), exp(j2π/5)}. The output of the DFT precoder is passed through an IFFT unit 550 and it is then mapped to a DFT-S-OFDM symbol 560.
As the previous operations are linear, their relative order may be inter-changed. Because the PUCCH transmission is assumed to be in one PRB which consists of NscRB=12 REs, there are 24 encoded HARQ-ACK bits transmitted in each slot (which includes 12 HARQ-ACK QPSK symbols) and a (32, OHARQ-ACK) RM code is punctured into a (24, OHARQ-ACK) RM code. The same or different HARQ-ACK bits may be transmitted in the second subframe slot. A RS is also transmitted in each slot to enable coherent demodulation of the HARQ-ACK signals. The RS is constructed from a CAZAC sequence 570, having a length of 12, which is passed through an IFFT 580 and mapped to another DFT-S-OFDM symbol 590.
The PUCCH structure of FIG. 5 can only support limited HARQ-ACK payloads without incurring a large coding rate because it can only support 24 encoded HARQ-ACK bits. For example, a single RM code can be used for HARQ-ACK payloads up to 10 bits and a dual RM code can be used for HARQ-ACK payloads between 11 and 20 bits. With a dual RM code, the mapping to successive elements of the DFT can alternate between elements from the output of a first RM code and elements from the output of a second RM code in a sequential manner, which is not shown for brevity. For HARQ-ACK payloads of more than 20 bits, convolutional coding can be used.
FIG. 6 illustrates a UE transmitter block diagram for HARQ-ACK signals in a PUCCH according to the related art.
Referring to FIG. 6, the HARQ-ACK information bits 605 are encoded and modulated by an encoder and modulator 610 and then multiplied with an element of the OCC 625 for the respective DFT-S-OFDM symbol by a mixer 620. The output of the mixer 620 is then precoded by a DFT precoder 630. After DFT precoding, sub-carrier mapping is performed by a sub-carrier mapper 640, under control of controller 650. Thereafter, the IFFT is performed by an IFFT unit 660, a CP is added by a CP inserter 670, and the signal is filtered by a filter 680 for time windowing, thereby generating a transmitted signal 690. For brevity, additional transmitter circuitry, such as a digital-to-analog converter, analog filters, amplifiers, and transmitter antennas are not illustrated in FIG. 6.
FIG. 7 illustrates a NodeB receiver block diagram for HARQ-ACK signals according to the related art.
Referring to FIG. 7, after receiving a Radio-Frequency (RF) analog signal and converting it to a digital received signal 710, the digital received signal 710 is filtered by a filter 720 for time windowing and a CP is removed by a CP remover 730. Subsequently, the NodeB receiver applies a FFT by a FFT unit 740, performs sub-carrier demapping by a sub-carrier demapper 750 under the control of a controller 755, and applies an Inverse DFT (IDFT) by an IDFT unit 760. The output of the IDFT unit 760 is then multiplied with an OCC element 775 for the respective DFT-S-OFDM symbol by a mixer 770. An adder 780 sums the outputs for the DFT-S-OFDM symbols conveying HARQ-ACK signals over each slot, and a demodulator and decoder 790 demodulates and decodes the summed HARQ-ACK signals over both subframe slots in order to obtain the HARQ-ACK information bits 795.
In TDD systems, as a UE needs to transmit HARQ-ACK information corresponding to potential TB receptions over multiple DL subframes, a DL Assignment Index (DAI) Information Element (IE), or DL DAI IE, VDAIDL, is included in each DL SA in order to assist the UE in determining there is a HARQ-ACK payload it should convey in a PUCCH. As the NodeB cannot predict whether there will be a DL SA for a given UE in future DL subframes, the VDAIDL is a relative counter which is incremented in each DL SA transmitted to a UE and starts from the beginning after the DL subframe is linked to the UL subframe of the HARQ-ACK signal transmission. Then, if the last DL SA is missed by the UE, the incorrect HARQ-ACK payload is transmitted which may cause incorrect understanding of at least some of the HARQ-ACK bits at the NodeB. In all following descriptions, the DL DAI IE is assumed to consist of 2 bits with the values of “00”, “01”, “10”, “11” respectively indicating VDAIDL=1, VDAIDL=2, VDAIDL=3, and VDAIDL=4.
FIG. 8 illustrates a setting for a DL DAI IE according to the related art.
Referring to FIG. 8, a bundling window consists of 4 DL subframes. In a DL subframe 0 810, the NodeB transmits a DL SA to a UE and sets the DL DAI IE value to VDAIDL=1. In DL subframe 1 820, the NodeB transmits a DL SA to the UE and sets the DL DAI IE value to VDAIDL=2. In DL subframe 2 830, the NodeB does not transmit a DL SA to the UE, and thus, there is no DL DAI IE value. In DL subframe 3 840, the NodeB transmits PDSCH to the UE and sets the DL DAI IE value to VDAIDL=3. If the UE misses the DL SA in the last subframe, it cannot know this event and an erroneous operation occurs as the UE cannot report a respective DTX or NACK.
If a UE does not detect a DL SA transmitted by the NodeB in a subframe other than the last one in a bundling window and detects a DL SA transmitted in a subsequent subframe in the same bundling window, it can infer from the DL DAI IE value of the latter DL SA the number of previous DL SAs it has missed. The total number of DL SAs a UE detects in a bundling window is denoted by UDAI. Therefore, a UE can know that it missed VDAI, lastDL−UDAI DL SAs where VDAI, lastDL is the DL DAI IE value in the last DL SA that the UE detects in a bundling window. The actual number of DL SAs the UE may actually miss can be larger than VDAI, lastDL−UDAI. This happens if the UE misses DL SAs after the last DL SA that it detects.
If a UE has a PUSCH transmission in an UL subframe where it also transmits HARQ-ACK information, the UE may transmit the HARQ-ACK information in the PUSCH. In order to avoid error cases where the UE has missed the last DL SA and in order to ensure the same understanding between the NodeB and the UE for the HARQ-ACK payload the UE transmits in the PUSCH, a DAI IE is also included in the UL SA so that there is an UL DAI IE to indicate the HARQ-ACK payload. If the PUSCH transmission is not associated with a UL SA, a UE assumes that there is a DL SA in every DL subframe in the bundling window.
As for the DL DAI IE, the UL DAI IE value VDAIUL is also assumed to be represented by 2 bits with the values of “00”, “01”, “10”, “11” respectively indicating VDAIUL=1, VDAIUL=2, VDAIUL=3, and VDAIUL=4 or 0. The UL DAI IE bits “11” map to VDAIUL=4 if the UE detects at least one DL SA in the bundling window; otherwise, they map to VDAIUL=0. In a case where the bundling window is larger than 4 subframes, the UL DAI IE value of “00” is assumed to indicate VDAIUL=5 if 1<UDAI≦5 or VDAIUL=9 if UDAI>5. Similar, the UL DAI IE value of “01” is assumed to indicate VDAIUL=6 if 2<UDAI≦6, the UL DAI IE value of “10” is assumed to indicate VDAIUL=7 if 3<UDAI≦7, and the UL DAI IE value of “11” is assumed to indicate VDAIUL=8 if 4<UDAI≦8.
In order to increase peak data rates, the NodeB can configure a UE with Carrier Aggregation (CA) of multiple cells to provide higher operating BWs. For example, in order to support communication over 60 MHz to a UE, a CA of three cells of 20 MHz each can be used. Assuming that the PDSCH in each cell conveys different TBs, the UE generates separate HARQ-ACK information for the respective TBs it receives in each cell. This is similar to single-cell TDD operations, where the UE generates separate HARQ-ACK information for the respective TBs it receives in each DL subframe for which the HARQ-ACK transmission is in the same UL subframe.
The NodeB, using higher layer signaling, can configure a set of C cells to a UE and activate a subset of A cells (A C) for PDSCH reception in a subframe, using for example Medium Access Control (MAC) signaling, however a UE may not transmit or receive in inactive cells. If a PDSCH activating or deactivating configured cells is missed, the UE and the NodeB may have a different understanding of the active cells. Moreover, in order to maintain communication, one cell with a DL/UL pair always remains active and is referred to as a Primary cell (Pcell). PUCCH transmissions from a UE are assumed to be only in its Pcell and HARQ-ACK information is conveyed only in a single PUSCH.
FIG. 9 illustrates a parallelization of the DL DAI IE design in FIG. 8 for operation with multiple DL cells according to the related art.
Referring to FIG. 9, a NodeB transmits DL SAs to a UE in 3 DL subframes in Cell 0 910 and sets the respective DL DAI IE values according to the number of DL SAs transmitted to the UE only for PDSCH transmissions in Cell 0 910. In a similar manner, the NodeB transmits DL SAs to the UE in 2 DL subframes in Cell 1 920 and 2 DL subframes in Cell 2 930 and sets the DL DAI IE values according to the number of DL SAs transmitted to the UE only for PDSCH transmissions in Cell 1 920 and Cell 2 930, respectively.
Alternate designs to the parallelization of the DL DAI design for PDSCH transmission in a single DL cell to multiple DL cells can be based on a joint DL DAI design across DL cells and DL subframes. For each DL subframe in the bundling window, the DL DAI counter operates first in the cell-domain before continuing to the next DL subframe in the bundling window.
FIG. 10 illustrates an operation of a joint DL DAI design across cells and DL subframes according to the related art.
Referring to FIG. 10, DL DAI IE values are shown only for DL subframes and configured DL cells where the NodeB transmits a DL SA to a UE. The DL DAI counter starts from DL subframe 0 in Cell 0 1010 and continues in the cell-domain DL subframe 0 for Cell 1 1020 and Cell 2 1030. After all DL SAs across the DL cells in DL subframe 0 are counted, the DL DAI counter continues sequentially for the remaining DL subframes in the bundling window in the same manner as used for the DL subframe 0. This DL DAI IE is also assumed to consist of 2 bits mapping to the values of VDAIDL=1, 2, 3, 0. After VDAIDL=3, the next value is VDAIDL=0 because VDAIDL, is computed modulo 4.
For a UE is configured for communication over multiple DL cells, the fundamental conditions to properly convey HARQ-ACK information to the NodeB remain the same as for single-cell communication. In other words, for transmission of an HARQ-ACK payload of OHARQ-ACK bits encoded with a (32, OHARQ-ACK) RM code in a PUSCH, the UE and the NodeB should have the same understanding of OHARQ-ACK. As the PUSCH transmission power is determined by assuming a data transmission and as the transmission powers of HARQ-ACK REs and data REs are the same, the HARQ-ACK reception reliability depends on the number of respective PUSCH REs which scales linearly with OHARQ-ACK as indicated in Equation (1). Therefore, whenever possible, OHARQ-ACK should not be a maximum value in order to avoid unnecessarily consuming PUSCH REs.
For HARQ-ACK transmission in a PUCCH, since a UE may miss some DL SAs, a common understanding for the HARQ-ACK payload between the UE and the NodeB is achieved only if the HARQ-ACK payload is always the maximum value of OHARQ-ACKmax=Nbundle·(C+C2) bits or, with spatial-domain bundling, OHARQ-ACKmax, bundle=Nbundle·C bits, where Nbundle is the size of the bundling window, c is the number of DL cells configured to the UE, and C2 is the number of configured DL cells where the UE is configured a PDSCH Transmission Mode (TM) conveying 2 TBs.
Using the maximum HARQ-ACK payload in a PUCCH does not create additional resource overhead. The UE may transmit a NACK or a DTX (in a case of tri-state HARQ-ACK information) for the TBs it did not receive, however, the NodeB already knows of the DL cells with no DL SA or PDSCH transmission to the UE and can use the knowledge that the UE transmits a NACK for each of those DL cells (a priori information) to improve the HARQ-ACK reception reliability. This is possible because a linear block code and QPSK are assumed to be used for the encoding and modulation of the HARQ-ACK bits, respectively, and the NodeB can consider as candidate HARQ-ACK codewords only the ones having a NACK (binary ‘0’) at the predetermined locations corresponding to cells without DL SA transmissions to the UE. Due to the implementation of the decoding process, the use of the a priori information would be impractical or impossible if a convolutional code or a turbo code was used for the encoding or if QAM was used for the modulation of the HARQ-ACK bits.
Although using the maximum HARQ-ACK payload for transmission in a PUCCH does not generate additional resource overhead, it often results in a larger transmission power than necessary for achieving the desired reception reliability. PUCCH transmissions with larger power than necessary increase UE power consumption and create additional interference degrading the reception reliability of signals transmitted by UEs in the same BW in other cells.
The PUCCH transmission power PPUCCH (i) in UL subframe i is assumed to be given as shown in equation (3), which is in units of decibels (dBs) per milliwatt (dBm).PPUCCH(i)=min{PCMAX,c,h(nHARQ-ACK(i))+F(i)}  Equation (3)where PCMAX,c is the maximum allowed UE transmission power in its Pcell, h(nHARQ-ACK(i)) is a monotonically increasing function of the nHARQ-ACK(i) HARQ-ACK information bits the UE assumes it is transmitting, and F(i) is a general function capturing all other parameters affecting PPUCCH (i) in UL subframe i. However, the present invention is not limited to the exact expression of h(nHARQ-ACK(i)), for example, it may be determined as h(nHARQ-ACK (i))=α·10 log 10(nHARQ-ACK(i), with α being a positive number, or h(nHARQ-ACK(i)) can be provided by a table indicating the transmission power as function of nHARQ-ACK(i). It is noted that the above expression does not account for possible multiplexing with the HARQ-ACK of additional information, such as a Service Request Indicator (SRI) used by a UE to indicate it has data to transmit. The key issue is for a UE to determine the proper nHARQ-ACK value. If nHARQ-ACK is too small the HARQ-ACK reception reliability is degraded. If nHARQ-ACK is too large, interference and UE battery consumption increase unnecessarily.
One possibility is for nHARQ-ACK(i) to be equal to the number of TBs the UE receives in a respective bundling window. This avoids excessive transmission power, but may underestimate the required transmission power as some DL SAs may be missed, thereby decreasing the HARQ-ACK reception reliability. Another possibility is to derive nHARQ-ACK(i) from the maximum HARQ-ACK payload as nHARQ-ACQ(i)=Nbundle·(C+C2). This ensures that the required HARQ-ACK reception reliability is always met, but will often result to the transmission power being excessively large. A variation of the second possibility is to consider only the number of activated cells A and the configured TM in each such cell. Then, nHARQ-ACQ(i)=Nbundle·(A+A2) where A2 is the number of activated cells with a configured TM conveying 2 TBs. However, excessive transmission power is again not avoided as not all active cells may transmit PDSCH to the UE in every DL subframe in the bundling window.
Therefore, there is a need to set the HARQ-ACK transmission power in a PUCCH while achieving the desired HARQ-ACK reception reliability in case of DL CA for a TDD system.
There is also a need to set the HARQ-ACK transmission power in the PUCCH while minimizing interference and UE power consumption in case of DL CA for a TDD system.
There is also a need to establish a common understanding between the UE and the NodeB about the correspondence between the HARQ-ACK information bits in the transmitted codeword and the respective cells and subframes in the case of a DL CA for a TDD system.
Finally, there is also a need to minimize the number of PUSCH REs allocated to HARQ-ACK transmission in case of DL CA for a TDD system.